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Window.event does not exist in Firefox. Different browsers have different event models. Firefox handles the events slightly different IE and Chrome and for that reason it has not worked. A work-around is to pass the event as an argument to the function:<a onclick="testfunction(event);">
//...
</a>

testfunction(e){
evt = e || window.event;
//...
}
or use a library like jQuery to avoid dealing with all the differences between the browsers.

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Comments

The answer give information why does the problem occur and that helps understanding it. But a little more description what "the function" does would have been very nice to understand the solution easily.